This invention relates to sound-absorbing acoustic ducts, and more particularly to acoustic ducts lined with material for absorbing spinning mode acoustical energy. As a typical application, the invention relates to noise suppressing linings for the inlet of an aircraft jet engine fan and/or compressor.
The sound pressure field generated by an axial flow compressor or fan operating with supersonic gas velocity relative to the rotor blade contains rotating pressure patterns or spinning modes caused primarily by shocks emanating from the leading edge of the rotor blades. It is these spinning modes that are transmitted through the duct and radiate objectionable noise to the outside.
It has been proposed to reduce such noise by lining the interior surface of the duct with acoustically absorbent material. One such lining which is tailored to suppress spinning mode sound is disclosed in U.S. application Ser. No. 503,031 filed by R. Mani on Sept. 3, 1974, now U.S. Pat. No. 3,937,590 and assigned to the assignee of this invention. The Mani application discloses that segmented acoustical treatment is effective to enhance scattering of spinning mode acoustic pressure fields to increase the higher order, attenuating type mode content for improved sound suppression in an acoustic duct of any cross section. As is known the duct has a cut-off property depending upon the tip speed of the rotating pressure patterns. Higher order mode number or lobe number pressure patterns have a tip speed less than the speed of sound and are attenuated, while lower order mode number pressure patterns have a tip speed greater than the speed of sound and are propagated. Assuming a cylindrical duct, the effect of attaching interrupted sections of strips of sound-absorbing material to the inner duct surface is to alter the modal distribution and provide an additional periodicity to which the sound field must conform. The acoustical energy is scattered to increase the ratio of higher order, attenuating modes. The number of equally spaced strips in the set is usually large, in any event several or more, and for the case where the noise source is a compressor with the rotor and stator blades, the number of strips exceeds or is a multiple of the number of first stage rotor blades. The lining disclosed in the Mani application comprises a plurality of peripherally spaced longitudinally extending strips of material which are effective to scatter spinning mode acoustic pressure fields to higher order attenuating modes. It has also been proposed to use such acoustically absorbent strips in a circumferentially extending manner. Such prior art structures have exhibited good noise reduction characteristics but there remains room for improvement, particularly under the ever more stringent noise requirements for newly certificated aircraft being set by Government regulatory agencies.